A drive device for a cable window lifter is described in German Patent Publication DE 31 45 277 C2, in which the cable drum comprises a first half for winding up the one cable end and a second half for winding up the other cable end. Ratchet teeth, which are in engagement with each other, are provided on the fronts of the cable drum halves facing each other. A spring presses the cable drum halves together and ensures that they turn in respect to each other but only after a defined difference in the transferable torque has been attained. For this purpose the first drum half is connected and fixed against relative rotation with the shaft of the drive, while the second drum half can be displaced on a cylindrical journal against the spring force in an axially limited manner and can then be turned in a circumferential direction.
The disadvantage of this device lies in that not only the so-called genuine cable slackness of a "relaxed" window lifter is compensated, wherein the drive moves over an appropriate dead angle making the typical ratcheting noise, but that an overload slackness is also removed. Such overload slackness can occur when the window lifter is moved against a stop with great force and the system becomes greatly deformed. Because the deformation is maintained, almost all components of the window lifter are put under heavy loads, which results in accelerated material fatigue and can even lead to system breakdowns.
A further device for compensating the length of the cable is described in Japanese Patent Publication JP 3-72552 B2. In accordance therewith, one of the Bowden tube supports of the window lifter is seated on the base plate supporting the cable drum so it is resilient in the direction of the cable. In this case the Bowden tube end comprises two parts in the area of the support which can be screwed together and through which the cable is guided. Both parts are loaded by a pressure spring which attempts to push the two parts, which can be screwed together, away from each other. The screw thread is embodied in such a way that, if cable slackness occurs, the two parts are unscrewed by the effect of the pressure spring until the cable is taut again. In the opposite direction detent teeth on the one screwable part, which are engaged with the end of the pressure spring embodied as a coil spring, prevent the two parts from entering each other. However, the above-described solution also has the disadvantage that it also removes cable slackness caused by an overload on the window lifter from the drive end, which results in a deformation of the mechanical system. A further disadvantage resides in that the proposed solution is only usable in connection with Bowden window lifters. This solution cannot be used in so-called open cable systems.